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It doesn't appear to be the PSU. The caps weren't bulged, but I replaced them just to be on the safe side, and the tivo acts the same way it did before I replaced the caps. I've checked the voltage on all of the rails, and the values are within tolerance for all of the rails. It appears that the tivo is attempting to boot when I get the flickering power light, I get output on the ttl port when I get the flickering light. I also get output when I get a solid green light. When I do get a successful boot, the tivo will run indefinitely so long as there isn't a power interruption. I'm stumped.

On a side note, I can't seem to get readable output from the ttl, it comes through as gibberish. I've tried both 115200 8N1 and 9600 8N1 port settings and I get jibberish using both a PL2303 usb setup, and a direct serial to ttl adapter. Should I expect to be able to read the output from the prom?

It's hard to say what's going on because it sounds so suspiciously like a PSU problem... I think the best thing to do is to test the unit with a known-good PSU. If you have another TiVo HD or a friend has one, you might try swapping the PSU's temporarily to see what's going on. Failing that, I would suspect a dodgy PROM connection but since you told me you were experiencing the symptom even prior to PROM socketing, I doubt that's the problem.